The present invention generally relates to resonant frequency identification, especially for resonant frequency identification through out-of-plane displacement detection.
The conventional optical interferometric surface profilometer has been developed mainly for static measurement of nano-scale three-dimensional surface profiles. It has been widely employed for measuring surface roughness and uniformity on semiconductor wafer, depth of laser mark, metal-bump size and co-planarity during flip chip bonding, size and height of spacers in liquid-crystal display panels, and surface profile of fiber end-face and micro optical devices. In recent years, vibratory measurement has been incorporated into the optical interferometric surface profilometer, thus widening its applications in observing and measuring the vibratory behavior of functional elements and thin-films in micro-electro-mechanical system (MEMS) and micro-opto-electro-mechanical system (MOEMS) industries.
When making dynamic measurement of objects in vibration, the laser-Doppler anemometry is the most commonly used. However, it is applicable for single-point measurement. Two-dimensional measurement of objects in vibration requires laser scanning point by point, which is more time-consuming and easily affected by environmental factors. Therefore, there measures may hardly obtain real-time full-field measurement of the surface profile of an object in vibrating mode.
Take Polytec MSA400, which is adapted from a Mach-Zehnder interferometer, for example. Polytec MSA400 comprises a laser-Doppler module, a white-light interferometric static three-dimensional profilometer and a stroboscopic in-plane displacement measurement module. When making out-of-plane displacement measurement, Polytec MSA400 first determines the optimal sampling points and paths of the area under test using its embedded software. Then the laser-Doppler module scans the pre-determined sampling points at a resonant frequency according to a set scanning frequency range and resolution. As a result, a resonant spectrum is obtained for each sampling point. All resonant spectrums are then combined to form a three-dimensional profile of out-of-plane displacement. However, the above-mentioned technology cannot perform full-field measurement, and the measurements obtained by Polytec contain no phase information.
According to the above, current systems and available technologies may have the following issues.                (1) Laser beams may cause damage to micro-devices under test due to overheating;        (2) The time-averaging approach may require manual identification of the resonant frequency;        (3) The laser source is relatively expensive and installation thereof is complicated, making it unsuitable for general MEMS/MOEMS systems.        
Moreover, U.S. Pat. No. 6,219,145 discloses a Michelson interferometer using an ultra-bright LED for dynamic measurement. However, during stroboscopic flashing, the displacement of the object under test has to be smaller than 1/20 of the wavelength of the light source; otherwise, fringes of the interference images may become blurred.
US Pat. Pub. No. 2005/027917 discloses a measurement and visualization technology for acquiring images of a vibrating object under test. However, the image information acquired concerns mainly marks, such as lines or paint, or a speckle pattern on the surface of the object under test. Resonant frequencies of the object under test are then identified using conventional image processing technologies.